The present invention relates to the field of matrix materials used in the manufacture of printing plates. More particularly, the present invention relates to a low fiber, high filler content matrix board used to form a matrix hold which is then employed in the manufacture of flexographic printing plates.
Matrix boards for use in forming flexographic printing plates are commonly manufactured by a paper making process on standard paperboard making equipment. Once the matrix board has been made, impressions are formed in the board by means of a master engraving plate. The impressioning is usually accomplished by pressing the matrix board against the master plate at relatively high pressure to form a mold. Engraved metal plates are commonly used as the masters from which the molds are formed; however, masters formed from rubber or photopolymer materials are also used.
Early matrix boards contained asbestos fibers which helped to reduce shrinkage of the board during the molding operation. However, since the asbestos fibers were present at relatively high loadings (30-60%) and were oriented by the paper machine in the machine direction, the resulting board presented highly directional shrinkage characteristics. That is, shrinkage along one axis of the board significantly exceeds that which occurs along the board's other axis.
Asbestos-free matrix boards became available in the early 1980's. In such matrix boards the asbestos fibers are replaced by cellulose fibers which, again, are present at relatively high loadings. Since cellulose fibers shrink more than asbestos and are also oriented in the machine direction during the manufacturing process, the asbestos-free boards are characterized by even higher directionality than boards formed with asbestos fibers.
Less directional matrix board materials made from cross-plied laminations of thin layers are known to those skilled in the art. However, because such materials contain a relatively high content of cellulosic fibers, they still exhibit significant shrinkage.
In addition to the shrinkage problem associated with matrix boards having a high fiber content, high mold pressures are generally required to mold faithful reproductions of the master plate into such boards. This is so because the high fiber content of these boards tends to reinforce the matrix and cause it to resist deformation. Accordingly, mold pressures of at least 300 psi are typical. Such high pressures tend not only to distort rubber and photopolymer masters but also compound the high shrinkage of such boards.
Accordingly, it is an object of the present invention to provide a low molding pressure matrix board material which exhibits low overall shrinkage and substantially less directional shrinkage.
It is another object of the present invention to provide such a matrix board material which can be formed on standard paperboard making equipment.
It is a still further object of the invention to provide a mold made from the above-described matrix board and a method for forming a flexographic printing plate from such a mold.